Use of a digital signal wireless communication using an infrared ray has been diversified, for example, it is used typically in a remote controller and recently in a mobile device provided with a communication device which is in conformity to an IrDA standard. In the communications, it is general to use an infrared light emission diode to perform data communications.
The infrared light emission diode requires approximately 50 mA as a driving current in sequentially emit light. However, in optical communications, a current which is several times to approximately 10 times as much as the driving current is subjected to pulse driving so as to secure transmission optical intensity, thereby enabling telecommunications. As a result, the light emission diode is made to excessively emit light, so that the light emission diode is deteriorated or damaged. In case where input of a transmission signal is continued for a certain time or more which causes the light emission diode to emit light, the driving current is stopped from being supplied to the light emission diode, so that it is possible to prevent the light emission diode from being deteriorated or damaged. Such technique is known, and is described as follows. FIG. 16 shows an arrangement of the conventional infrared transmitter circuit.
As shown in FIG. 16, the conventional infrared transmitter circuit is arranged so that: when a power source circuit PS11 turns ON in response to input of a transmission signal, a current is supplied from the power source circuit PS11 to a constant current source CS11. Thus, when a current flows from the constant current source CS11 to a base of a transistor Q11, the transistor Q11 turns ON, so that the transmission signal flows to bases of transistors Q12 and Q13 via the transistor Q11. Therefore, the transistors Q12 and Q13 turn ON only at a time when a level of the transmission signal is high. The transistors Q11 to Q13 constitute a current mirror circuit, so that an output current Io, whose value is the same as that of a constant current flowing from the constant current source CS11 to the transistor Q12, flows from a power source of a voltage Vcc to the transistor Q13 via a light emission diode LED 11.
Further, as shown in FIG. 17, when a certain time tx passes after the transmission signal has been inputted (high level), a timer T causes the constant current source CS11 to stop. Thus, the transistor Q12 turns OFF, so that also the transistor Q13 turns OFF. As a result, the output current Io is stopped from being supplied to the light emission diode LED11.
An example of a document which recites such a technique that the light emission diode is protected on the basis of the foregoing time management is Japanese Unexamined Patent Publication No. 50785/1983 (Tokukaisho 58-50785)(Publication date: Mar. 25, 1983).
However, the circuit shown in FIG. 16 uses a high level of the transmission signal as a power source of the power source circuit PS11. Thus, as shown in FIG. 17, also after the output current Io of the light emission diode LED11 becomes 0 with the timer T indicating the certain time tx, the power source current Icc flows to the power source circuit SP11 until the transmission signal becomes “0”, so that the current consumption does not become 0. As a result, in case where the infrared transmitter circuit is installed on a device, such as a mobile device, which operates on the basis of a battery, a time in which the device can be used is short.
For example, according to the aforementioned IrDA standard, it is necessary that the communication element can be connected directly to a serial port of a personal computer. The light emission diode which performs transmission under a high level condition emits light, but an output of the serial port of the personal computer is a high level when it is not controlled. Thus, when the serial port is not controlled by a computer, the serial port is under a condition that causes the light emission diode to emit light. However, in the foregoing circuit, the driving current is stopped from being supplied to the light emission diode LED11 after a certain time has passed, but a current always flows to the power source circuit SP11 in this while. Thus, in a battery-driven device such as a lap top personal computer or a mobile phone provided with the IrDA communication element, the power consumption increases, so that a time in which the device can operate is short.